Semiconductor laser devices have been produced but they have had various drawbacks, such as difficulty of production or, when produced, excessive leakage current resulting from unwanted contact between certain semiconductor layers in the device.
FIG. 6 shows a cross-sectional illustration of a typical prior art semiconductor laser device described in Japanese Laid-open Patent Publication No. 61-204994. That structure is based on a p type semiconductor substrate 1 having an active layer 2 grown on the substrate, and an n cladding layer 3 grown on the active layer. After the two layers are grown on the substrate during a first liquid phase epitaxial growth process, a pair of channels are etched to produce a mesa structure generally designated at 1' in which a central stripe carrying the active layer and n cladding layer is disposed above the substrate. In order to confine the current within the active layer 2, additional p-n layers are grown on either side of the mesa portion. In the illustrated embodiment, a p type embedded layer 4 is grown over the substrate (as well as over the portions of the active layer 2 and n cladding layer 3 near the edges of the device). Following growth of the p type embedded layer 4, additional layers are grown including an n type current blocking layer 5 and a p type current blocking layer 6. Finally, an n type cladding layer 7 is grown over the entire top surface including the cladding layer 3 and the p type current blocking layer 6 to form a substantially flat portion for receipt of an electrode (not illustrated).
In operation, when a voltage is applied between the p type substrate 1 and the n type cladding layer 7, the holes and electrons which are the carriers of the respective layers are injected into the active layer 2. When the injection current reaches a predetermined (preferably fairly low) level, laser oscillation occurs and light is emitted. The injected carriers are concentrated in the active layer 2, in part by the automatically reversed biased p-n junction comprised of the current blocking layers 5, 6. By virtue of the barrier created by the reverse biased p-n junction 5, 6, current is largely confined within the active region, and the semiconductor laser device should operate at a high efficiency and with low leakage or idle current.
In practice, however, such a laser device does not attain the expected high efficiency because of leakage currents which are created by virtue of the juxtaposition of certain layers in the device. More particularly, in construction of such a device there is contact, and therefore an electrical connection, usually created between the n type current blocking layer 5 and either the n type cladding layer 3 or the n type cladding layer 7, and that connection results in a leakage current through the device. The leakage or idle current which flows in the device is illustrated by the arrows shown in FIG. 6, and is relatively high considering that the resistivity of the n type layer is less than that of the p type layer by about an order of magnitude. Furthermore, since the area of the p-n junction produced between the p type embedded layer 4 and the n type current blocking layer 5 is large, the total leakage current from the n blocking layer to one of the n cladding layers will be quite high. Indeed, it has been found, that when the semiconductor laser is produced by liquid phase epitaxy, the n type current blocking layer 5 and one or the other of the n cladding layers 3, 7 will be connected in almost all cases. Although the width of this connection is generally small, such as 0.1 .mu.m, a large leakage current will flow for the reasons described above.